1. Field of the Invention
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, an image forming apparatus using an intermediate transfer method in which a plurality of toner images formed on a plurality of image bearing members are transferred onto an intermediate transfer member and then to a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In known electrophotographic image forming apparatuses, toner images of yellow (Y), magenta (M), cyan (C), and black (K) are formed on photosensitive drums of the respective colors and transferred onto an intermediate transfer member such as an intermediate transfer belt such that they are superimposed one atop the other, thereby forming a composite toner image on the intermediate transfer member in a process known as a primary transfer process. Subsequently, the composite toner image is transferred secondarily onto a recording medium in a secondary transfer nip at which the intermediate transfer member and a nip forming member contact. This process is known as a secondary transfer process.
This type of image forming apparatuses may be equipped with an additional photosensitive drum for forming a special color toner image such as a transparent toner image, in addition to the photosensitive drums for black and primary colors yellow, and magenta. The transparent toner image is formed on the photosensitive drum to add a glossy effect to a certain area of the composite toner image. Generally, the transparent toner image is transferred primarily onto the intermediate transfer member first. Subsequently, the toner images of yellow, magenta, cyan, and black are transferred one atop the other on the transparent toner image on the intermediate transfer member. In the secondary transfer process, the composite toner image on the transparent toner image is transferred onto a recording medium. Accordingly, the transparent toner image is on top of the composite toner image to provide the glossy effect on the color image.
With diversified color expression in recent years, a solid transparent toner image is formed to enhance the glossy effect on a solid image. In addition to the solid image, a fine line, an outline of an image, and a character image are expressed with the transparent toner. Present inventors performed experiments in which a fine-line image and a character image were formed with the transparent toner on top of a solid color background image formed with toner of a single color or a composite of at least two colors among yellow, magenta, cyan, and black.
As shown in FIGS. 1 and 2, an image density of the solid color background image around the fine-line image and the character image formed with the transparent toner dropped significantly. In other words, inadequate transfer of toner, also known as dropouts, occurred around the line image and the character image. The inadequate transfer of toner occurred because the recording medium did not contact tightly around the fine-line image and the character image formed on the solid color background image. More specifically, because the portion of the solid color background image on which the fine-line image and the character image were formed with the transparent toner was higher than other areas without the fine-line image and the character image. Due to the height difference, the recording medium could not contact tightly around the fine-line image and the character image with the transparent toner in the solid color background image. As a result, the color toner such as yellow, magenta, cyan, and black could not transfer well from the intermediate transfer member onto the recording medium. Thus, inadequate transfer of toner occurred around the line image and the character image.
Although it was not as visible as the dropouts around the fine-line image and the character image, the present inventors also found inadequate transfer of toner around the image supplied solidly with the transparent toner. However, such an area around the image formed with the transparent toner may be a non-image formation area to which no toner is supplied and an image area having multiple colors such as a photo image in which such dropouts are not visible. Improper transfer of toner such as dropouts appeared especially noticeable when an image including a fine-line image and a character image was formed on a solid color background image. Moreover, dropouts also occurred when forming a fine-line image and a character image with color toner on a solid color background image.
In view of the above, in one approach, a superimposed bias in which an alternating current (AC) component is superimposed on a direct current (DC) component is employed as a secondary transfer bias to form a transfer electric field in the secondary transfer nip. In order to facilitate an understanding of the related art and of the novel features of the present invention, with reference to FIGS. 3 through 5, a description is provided of principles of toner movement when applied with the superimposed bias according to an experiment performed by the present inventors.
FIG. 3 illustrates movement of toner in the transfer nip in a test machine at the beginning of transfer. As illustrated in FIG. 3, a polyimide belt 214 of the test machine serves as an intermediate transfer member that carries a color toner image on its image bearing surface. The color toner image includes a fine-line image formed of toner particles Ty of yellow toner and a solid color background image formed of toner particles of Tc of cyan toner. The fine-line image and the solid color background image are superimposed one atop the other. Because the transparent toner particles are difficult to see, in the experiment, the toner particles Ty instead of the transparent toner particles were used to form the fine-line image. The toner particles Ty and Tc were negatively chargeable toner particles.
A portion of the color image including the fine-line image with the toner particles Ty and the solid color background image with the toner particles Tc superimposed on the fine-line image contacted tightly a transparent substrate 210. In the experiment, the transparent substrate 210 corresponds to a recording medium.
As illustrated in FIG. 3, the portion of the solid color background image of the toner particles Tc superimposed on the toner particles Ty of the fine-line image contacted tightly the transparent substrate 210. By contrast, there was a gap between the transparent substrate 210 and the portion of the solid color background image where the toner particles Tc were not superimposed on the toner particles Ty of the fine-line image.
In this state, when the secondary transfer bias consisting of the DC component having a positive polarity superimposed on the AC component was applied to the polyimide belt 214, some toner particles Tc separated from the toner layer of toner particles Tc (hereinafter referred to as toner layer C) that had not contacted the transparent substrate 210. As a result, the separated toner particles Tc moved back and forth between the toner layer C and the transparent substrate 210. The cycle of the back-and-forth movement of the toner particles was in sync with the cycle of the AC component of the secondary transfer bias.
As illustrated in FIG. 3, in the first cycle, only a small amount of toner particles Tc separated from the toner layer C. The separated toner particles Tc made one back-and-forth movement between the toner layer C and the transparent substrate 210. In this process, the returning toner particles Tc collided with other toner particles Tc remaining in the toner layer C, thereby reducing adhesion of the other toner particles to the toner layer C or to the transparent substrate 210. As a result, in the next cycle, a larger amount of toner particles than in the previous cycle separated from the toner layer C, as illustrated in FIG. 5.
Subsequently, the separated toner particles Tc made one back-and-forth movement between the toner layer C and the transparent substrate 210. Again, the returning toner particles Tc collided with other toner particles remaining in the toner layer C, thereby enhancing separation of the toner particles Tc from the toner layer. As a result, in the next cycle, an even larger amount of toner particles than in the previous cycle separated from the toner layer.
With this configuration, as the toner particles Tc moved back and forth, the amount of toner particles separating from the toner layer increased, and lastly, a sufficient amount of toner particles Tc moved to the transparent substrate 210 which corresponds to a recording medium. A sufficient image density was obtained around the fine-line image in the solid color background image.
Although advantageous and generally effective for its intended purpose, application of the superimposed bias as the secondary transfer bias causes scattering of toner easily. As the toner particles are moved back and forth between the intermediate transfer member and the recording medium in the secondary transfer nip, the toner particles scatter and stick undesirably to a non-image formation area of the recording medium. In a case in which the secondary transfer bias consisting of the superimposed bias for preventing dropouts is applied even when forming an image without a fine-line and a character image on a solid color background image, scattering of toner still occurs.
In view of the above, there is demand for an image forming apparatus that is capable of preventing inadequate transfer of toner while preventing scattering of toner.